"We've got a situation with some north-westerlies aloft and southerlies pushing in," MetService meteorologist Tam Pletzer said.
"We're getting snow levels pretty low, at 200 metres in Canterbury at the moment with possible flurries getting down to 100 [metres]."
People have reported seeing snow near sea level in Temuka, Canterbury.
MetService issued snow warnings for the Lewis, Arthurs, Porters, Lindis and Haast passes, as well as for the Crown Range and Milford roads.
Queenstown Lakes District Council is requiring chains to be fitted on vehicles driving over the Crown Range Rd, and for them to be carried on a number of other routes.
Snow was falling around Wakatipu Basin but warm roads in the lower parts of the basin meant it wasn't settling at this stage, a statement from the council said.
Visibility is low, motorists are warned of is debris on roads after strong overnight winds.
"Our crews are working to clear this but please take extra care and slow down," the statement said.
"Heavy rain has also caused some minor rockfalls through the Kawarau Gorge and there's heavy snow further afield too.
"If you're planning to head over the Lindis Pass or to Haast, make sure you have your chains and you know how to fit them if required."
The snow and heavy rain near Aoraki/Mt Cook overnight helped firefighters battling to slow a huge scrub fire burning since Sunday, dampening flammable materials in the fire's path.
The Milford Road is closed from East Gate to West Gate due to snow.
The Transport Agency said there was snow on the Te Anau side of the Homer Tunnel but none on the western side. The road was expected to reopen before 10am.
There is snow and ice on State Highway 79 between Fairlie and Rangitata and the agency urged drivers to take extra care
Care is also needed on the Devil's Staircase (Kingston to Frankton) and between Wanaka and Cardrona. Chains must also be carried.
"Please follow any instructions on the information boards or checkpoint crews."
In Central Otago, there's snow and ice in the Manuherikia and Maniototo in freezing temperatures this morning. Crews were working to clear the roads, but care is needed, the district council said.
Danseys Pass Road is closed (from the gates past the Danseys Pass Hotel) due to snow and ice.
Meanwhile overnight rain in Roxburgh, Alexandra and Cromwell has caused surfacing flooding and windblown debris in areas.
NIWA meteorologist Chris Brandolino said overall spring was expected to be warmer than usual for all of Aotearoa.
Rainfall is also expected to be near or below normal in the South Island which could be an issue for the fire season, he said.
- Additional reporting ODT Online and RNZ
